Method of fabricating semiconductor laser

ABSTRACT

In fabricating a semiconductor laser  10  with an oscillation wavelength of 770 to 810 nm, impurities are introduced into an MQW active layer  16  near a light emitting facet of the laser to form a disordered region constituting a window layer  20.  Pumped light is applied to the window layer  20  to generate photo luminescence whose wavelength λ dpl (nm) is measured. A blue shift amount λ bl (nm) is defined as the difference between the wavelength λ apl (nm) of photo luminescence generated by application of pumped light to the active layer  16  on the one hand, and the wavelength λ dpl (nm) of photo luminescence from the window layer  20  under pumped light irradiation on the other hand. The blue shift amount λ bl is referenced during the fabrication process in order to predict COD levels of semiconductor laser products.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor laser device anda method for fabricating such a device. More particularly, the inventionrelates to a semiconductor laser device for use in optical dataprocessing and to a method for fabricating that device.

[0003] 2. Description of the Related Art

[0004] A trend in recent years has been for CD-R/RW drives to run atincreasingly higher speeds than before. That trend has entailed agrowing need for semiconductor lasers of the 780 mm band, which are usedby the high-speed drives; to provide greater output. A major constrainton getting the semiconductor laser to be more highly power is adegradation of its light emitting facet. This type of degradation,called COD (catastrophic optical damage) degradation, stems from defectsin the vicinity of the light emitting facet causing optical absorption.

[0005] One way to reduce the COD degradation at the light emitting facetis by having a window structure laser with a wide band gap region, i.e.,a region where no light absorption takes place, formed on the lightemitting facet. One such solution is described illustratively in theSharp Technical Report (a Japanese publication), No. 50, Sept. 1991, pp.33-36.

[0006]FIG. 14 is a partial perspective view of a semiconductor laserhaving a conventional window structure. FIGS. 15A and 15B are partialperspective views showing steps of a method for fabricating thesemiconductor laser with the conventional window structure.

[0007] In FIG. 14, reference numeral 100 represents a semiconductorlaser, and 102 denotes an n-type GaAs substrate (in the description thatfollows, the symbol “n-” standsfor the n-conductivity type, “p-” for thep-conductivity type, and “i-” for an intrinsic semiconductor). Referencenumeral 104 stands for an n-Al_(0.5)Ga_(0.5)As lower clad layer; 106 foran MQW active layer having an i-Al_(0.1)Ga_(0.5)As well layer; 108 for ap-Al_(0.5)Ga_(0.5)As first upper clad layer; 110 for an n-AlGaAs currentblocking layer; 112 for a p-AlGaAs second upper clad layer; 114 for ap-GaAs contact layer; 116 for an i-Al_(0.5)Ga_(0.5)As window layer witha band gap greater than that of the MQW active layer 106; and 118 for anelectrode.

[0008] The conventional method for fabricating the semiconductor lasersketched above will now be outlined. In FIG. 15A, the lower clad layer104, MQW active layer 106 and first upper clad layer 108 are epitaxiallygrown on the n-GaAs substrate 102. With a ridge produced by etching, thecurrent blocking layer 110 is selectively grown. The second upper cladlayer 112 and the contact layer 114 are then formed over the ridge andcurrent blocking layer 110. The result of these steps is shown in FIG.15A.

[0009] Thereafter, the back of the n-GaAs substrate 102 is shaved to athickness of about 100 μm. Laser facets are cleaved and the window layer116 is formed by crystalline growth. The result of this process isillustrated in FIG. 15B. Forming the electrode 118 on the structurecompletes the semiconductor laser of FIG. 14.

[0010] On the conventional semiconductor laser 100 constituted asoutlined above, the window layer 116 is formed by crystalline growth onthe cleaved surface following cleavage of the laser facts. Thisconventional process tends to be complicated because the window layer116 and electrode 118 need to be formed after the cleaving step.

[0011] Japanese Patent Publication No. 2827919 discloses a method forforming a window structure. The method includes forming a first upperclad layer on an MQW active layer, to be topped subsequently with an ionimplantation mask pattern, forming the window structure by getting theMQW active layer disordered in the vicinity of the laser facet by meansof impurity implantation at a low energy level. According to thedisclosed method, the degree of the disorder must be controlledprecisely, otherwise the window effect will not occur, resulting in asemiconductor laser degrading during use.

SUMMARY OF THE INVENTION

[0012] The present invention has been made to overcome theabove-described drawbacks and disadvantages of the related art.Therefore, it is an object of the present invention to provide a highlyreliable semiconductor laser device offering a significantly consistentimmunity to COD degradation.

[0013] According to one aspect of the invention, there is provided asemiconductor laser device with an oscillation wavelength of 770 to 810nm, comprising: a semiconductor substrate of a first conductivity type;a first clad layer of the first conductivity type disposed on thesemiconductor substrate; an active layer of a quantum well structuredisposed on the first clad layer; a first second-clad layer of a secondconductivity type disposed on the active layer; a disordered regionformed near a laser resonator facet by introducing impurities from asurface of the first second-clad layer into the layers including theactive layer on the semiconductor substrate; and an optical waveguideincluding a second second-clad layer of the second conductivity typedisposed on the surface of the first second-clad layer in a manneropposite to the active layer in the disordered region across the firstsecond-clad layer, the optical waveguide extending in a resonatorlengthwise direction; wherein, if λ dpl is assumed to denote in nm thewavelength of photo luminescence generated by application of pumpedlight to the disordered region and λ apl to represent in nm thewavelength of photo luminescence generated by application of pumpedlight to the active layer, and if a blue shift amount λ bl in nm isdefined as equal to λ apl−λ dpi then the blue shift amount λ bl meets acondition of λ bi≧20.

[0014] Accordingly, when the active layer is disordered so as toconstitute the window layer, the semiconductor laser is considered tohave acquired an improved COD level. This makes it possible to fabricatesemiconductor laser devices with consistently and appreciably limitedvariations of immunity from COD degradation.

[0015] Another object of the invention is to provide a method insimplified steps for fabricating at high yield rates a semiconductorlaser device offering an appreciably consistent immunity from CODdegradation.

[0016] According to another aspect of the invention, there is provided asemiconductor laser device fabricating method including the steps of:firstly forming a first clad layer of a first conductivity type, anactive layer of a quantum well structure, and a first second-clad layersuccessively on a semiconductor substrate of the first conductivitytype; secondly forming on a surface of the first second-clad layer amask pattern for impurity implantation having an opening in a regionwhere a resonator facet of a semiconductor laser device is expected tobe formed; thirdly disordering the active layer near the resonator facetby introducing impurities with the mask pattern used as a mask; fourthlyapplying pumped light to the disordered region to generate photoluminescence therefrom, and measuring a wavelength of the photoluminescence as a basis for predicting a level of COD degradation;fifthly forming a second second-clad layer of the second conductivitytype on the surface of the first second-clad layer after removing themask pattern for impurity implantation; sixthly forming on a surface ofthe second second-clad layer a stripe-shaped mask pattern in a manneropposed to the disordered active layer across the first and the secondsecond-clad layer, the stripe-shaped mask pattern extending in aresonator lengthwise direction; and seventhly forming an opticalwaveguide including the second second-clad layer with the stripe-shapedmask pattern used as a mask.

[0017] Accordingly, the inventive method allows levels of CODdegradation to be predicted halfway through the process of semiconductorlaser fabrication. This permits high-yield, low-cost fabrication ofsemiconductor laser devices in simplified steps.

[0018] Other objects and advantages of the invention will becomeapparent from the detailed description given hereinafter. It should beunderstood, however, that the detailed description and specificembodiments are given by way of illustration only since various changesand modifications within the scope of the invention will become apparentto those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a front view of a light emitting facet of asemiconductor laser in accordance with one embodiment of the invention;

[0020]FIG. 2 is a cross sectional view of the semiconductor laser takenon line II-II in FIG. 1;

[0021]FIG. 3 is a front view of the light emitting facet of thesemiconductor laser in one step of fabricating a semiconductor laser inaccordance with one embodiment of the invention;

[0022]FIG. 4 is a cross sectional view taken on line IV-IV in FIG. 3;

[0023]FIG. 5 is a front view of the light emitting facet of thesemiconductor laser in another step of fabricating a semiconductor laserin accordance with one embodiment of the invention;

[0024]FIG. 6 is a cross sectional view taken an line VI-VI in FIG. 5;

[0025]FIG. 7 is a schematic view showing illustratively how photoluminescence of a semiconductor laser in accordance with one embodimentof the invention is measured;

[0026]FIG. 8 is a front view of the light emitting facet of thesemiconductor laser in another step of fabricating a semiconductor laserin accordance with one embodiment of the invention;

[0027]FIG. 9 is a cross sectional view taken on line IX-IX in FIG. 8;

[0028]FIG. 10 is a front view of the light emitting facet of thesemiconductor laser in another step of fabricating a semiconductor laserin accordance with one embodiment of the invention;

[0029]FIG. 11 is a front view of the light emitting facet of thesemiconductor laser in another step of fabricating a semiconductor laserin accordance with one embodiment of the invention;

[0030]FIG. 12 is a graphic representation of relations between photoluminescence wavelengths (nm) of the window layer in a semiconductorlaser in accordance with one embodiment of the invention on the onehand, and COD levels (mW) of the semiconductor laser on the other hand;

[0031]FIG. 13 is a graphic representation of relations between CODlevels of a semiconductor laser in accordance with one embodiment of theinvention on the one hand and its blue shift amount on the other hand;

[0032]FIG. 14 is a partial perspective view of a semiconductor laserhaving a conventional window structure; and

[0033]FIGS. 15A and 15B are partial perspective views showing steps of amethod for fabricating the semiconductor laser with the conventionalwindow structure.

[0034] In all figures, the substantially same elements are given thesame reference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] A semiconductor laser having an oscillation wavelength of 770 to810 nm according to the invention has its COD levels predictedillustratively as follows. Impurities are introduced (i.e., implanted ordiffused) into the MQW active layer near the light emitting facet of thesemiconductor laser to form disordered regions constituting a windowlayer. Pumped light is emitted to the window layer to have it producephoto luminescence whose wavelength λ dpl (nm) is measured. A blue shiftamount λ bl (nm) is defined as the difference between the wavelength λapl (nm) of photo luminescence generated by application of pumped lightto the active layer without a window layer on the one hand, and thewavelength λ dpl (nm) of photo luminescence from the window layer on theother hand. The blue shift amount λ bl is referenced during the processin order to predict COD levels of the end product.

[0036]FIG. 1 is a front view of a light emitting facet of asemiconductor laser embodying the invention. FIG., 2 is a crosssectional view of the semiconductor laser taken on line II-II in FIG. 1.

[0037] In FIGS. 1 and 2, reference numeral 10 stands for a semiconductorlaser having an oscillation wavelength of 770 to 810 nm; 12 for ann-GaAs-substrate; 14 for an n-Al_(0.5)Ga_(0.5)As lower clad layer as afirst clad layer disposed on the n-GaAs substrate 12; 16 for an MQWactive layer which is disposed on the lower clad layer 14 and whichcomprises an i-Al_(0.1)Ga_(0.5)As well layer as well asAl_(0.3)Ga_(0.5)As barrier and guide layers; and 18 for ap-Al_(0.5)Ga_(0.5)As first upper clad layer as one second-clad layerdisposed on the MQW active layer.

[0038] Reference numeral 20 denotes a disordered region constituting awindow layer disposed in a light emitting region in the vicinity of aresonator facet of the semiconductor laser 10. This is a region with itsband gap widened when impurities are introduced by ion implantation ordiffusion from the surface of the first upper clad layer 18 so as to getthe MQW active layer disordered. Thus constituted, the region staystransparent to laser emissions.

[0039] More specifically, when the well layer, barrier layer and guidelayer making up the original MQW active layer 16 are disordered, an Alcomposition ratio of the well layer becomes greater than that of theoriginal well layer. That in turn increases the band gap.

[0040] Reference numeral 22 denotes a p-Al_(0.5)Ga_(0.5)As second upperclad layer as another second clad layer disposed on the first upper cladlayer 18 and window layer 20. Numeral 24 represents a p-GaAs contactlayer disposed on the second upper clad layer 22. The second upper cladlayer 22 and contact layer 24 make up a ridge 25 as a waveguide in theoptical waveguide direction. The contact layer 24 is formed at the topof the ridge 25.

[0041] Reference numeral 26 stands for an insulating film such as anSiON film which blocks currents and which is formed on the surface ofthe second upper clad layer 22 on both sides of the ridge 25 and in itsvicinity. Numeral 28 denotes a p-type electrode disposed on the surfaceof the semiconductor laser, and numeral 30 represents an n-typeelectrode disposed on the back of the n-GaAs substrate 12.

[0042] A semiconductor laser fabricating method embodying the inventionwill now be described. FIGS. 3, 5, 8, 10 and 11 are front views of thelight emitting facet of the inventive semiconductor laser in differentstep of the inventive fabricating method. FIG. 4 is a cross sectionalview taken on-line IV-IV in FIG. 3; FIG. 6 is a cross sectional viewtaken on line VI-VI in FIG. 5; and FIG. 9 is a cross sectional viewtaken on line IX-IX in FIG. 8. Cross sectional views taken on line X-Xin FIG. 10 and on line XI-XI in FIG. 11 are the same as that in FIG. 9.FIG. 7 is a schematic view showing how photo luminescence of theinventive semiconductor laser is illustratively measured.

[0043] Referring first to FIGS. 3 and 4, the lower clad layer 14, MQWactive layer 16, and first upper clad layer 18 are epitaxially grown onthe n-GaAs substrate 12. Referring to FIGS. 5 and 6, a resist is formedon the surface of the first upper clad layer 18. A mask pattern 40 isformed together with an opening 42 for constituting the window layer 20at the laser resonator facet. Impurities are introduced from above themask pattern 40 through the opening by diffusion or ion implantation.Annealing is carried out to disorder the MQW active layer 16 to form thewindow layer 20. Arrows in FIGS. 5 and 6 indicate the direction in whichdiffusion or ion implantation is carried out. Ion implantation, ifadopted, is executed at an acceleration voltage of 50 keV to 150 keVwith a dose of about 1×10¹³ to 1×10¹⁵/cm².

[0044] The resist 40 is removed, and the wafer is annealed in order todisorder the MQW 16. After the annealing, pumped light is applied fromabove the wafer so that photo luminescence from the window layer 20 ismeasured. FIG. 7 shows how photo luminescence is illustrativelymeasured. In FIG. 7, reference numeral 44 stands for pumped light; 46for photo luminescence emitted from the window layer 20; and 48 for ameasuring instrument that measures levels of photo luminescence.

[0045] After the measurement, the second upper clad layer 22 is formedon the first upper clad layer 18 and window layer 20. The contact layer24 is disposed on the second upper cad layer 22. FIGS. 8 and 9 depicthow the layers are formed.

[0046] A stripe-shaped resist pattern (not shown) such as a resist is,then formed in the resonator lengthwise direction of the light emittingregion. With the resist pattern used as a mask, etching is carried outto shave the second upper clad layer 22 to a predetermined thickness,thereby forming the ridge 25. FIG. 10 shows how this step is performed.

[0047] The top of the ridge 25 is then truncated. Insulating films 26are formed along both sides of the ridge 25 and over the second upperclad layer 22 around the ridge 25. The insulating films 26 are providedso as to block currents. FIG. 11 depicts how this step is done.

[0048] Thereafter, the p-type electrode 28 is formed on the contactlayer 24 and insulating films 26 at the top of the ridge 25. The back ofthe n-GaAs substrate 12 is ground to about 100 μm, whereby the n-typeelectrode 30 is formed at the substrate back. Finally, cleaving iscarried out to complete the semiconductor laser as shown in FIGS. 1 and2.

[0049] The window structure laser formed by the above-described sequenceof steps is characterized in that the window layer and electrode areformed during the wafer process. Compared with conventionalsemiconductor lasers with their window layer formed after cleaving, theinventive semiconductor laser is easy to mass-produce.

[0050] Below is a description of how to measure photo luminescence(called PL hereunder) emitted from the window layer 20 upon irradiationof pumped light from above the wafer.

[0051]FIG. 12 is a graphic representation of relations between PLwavelengths (nm) of the window layer 20 in the inventive semiconductorlaser on the one hand, and COD levels (mW) of the semiconductor laser onthe other hand. In other words, FIG. 12 shows relations between levels fthe optical output destroying the light emitting facet of thesemiconductor laser on the one hand, and PL wavelengths stemming fromthe semiconductor laser window layer on the other hand.

[0052] Samples A, B and C represent semiconductor lasers of the samestructure but fabricated under partially different conditions. In eachgroup of samples of the same type, as illustrated in FIG. 12, theshorter the PL wavelength, the higher the COD level. Why the COD leveltends to be higher the shorter the PL wavelength is because less lightis absorbed near the resonator facet as the band gap becomes greater inkeeping with higher degrees of disorder in the window layer.

[0053] In FIG. 12, the COD level is assumed to be represented by Pcod(mW) and the PL wavelength of the window region by A dpl (nm). Pcod islinearly proportional to λ dpl. That is,

Pcod=fl(λ dpl)  (1)

[0054] where, fl(x) is a linear function.

[0055] With this semiconductor laser, λ dpl is also linearlyproportional to Pcod, so that the following equation is readily derivedfrom the equation (1) above:

λdpl=gl(Pcod)  (2)

[0056] If Pcod is defined as a value made of a required laser output P0and a power margin P1, then

Pcod=P 0+P 1  (3)

[0057] In that case, the PL wavelength identifying the semiconductorlaser having Pcod is obtained as λ dpl. With this value used as areference, the semiconductor laser can be checked for compliance withCOD level requirements just after the window layer 20 is formed. Thatis, when the PL wavelength λ dpl (nm) of the window layer 20 ismeasured, it is possible consistently to produce semiconductor lasers ofhigh reliability at high yield rates, with the presence of the windowlayer 20 working against COD degradation.

[0058] Semiconductor lasers with no window layer and having the samestructure as those discussed in connection with FIG. 12 are known tohave the COD level of about 200 mW. As shown in FIG. 12, the PLwavelength is 750 nm or less when the COD level is 200 mW or higher.

[0059] It follows that if semiconductor lasers have an MQW active layerwith oscillation wavelengths of 770 to 810 nm and if they have a windowlayer 20 such that the PL wavelength is 750 nm or less, then thesesemiconductor lasers turn out to have improved COD levels at leastthanks to the effect of the window layer 20.

[0060]FIG. 13 is a graphic representation of relations between CODlevels of the inventive semiconductor laser on the one hand and its blueshift amount on the other hand.

[0061] The blue shift amount λ bl (nm) is defined here as the differencebetween the wavelength λ apl (nm) of photo luminescence (PL) generatedby application of pumped light to an active layer yet to be disorderedon the one hand, and the wavelength λ dpl (nm) of photo luminescencefrom the window layer 20 on the other hand. Given the definition, therelations in FIG. 12 are rearranged into what is shown in FIG. 13.

[0062] Since semiconductor lasers with oscillation wavelengths of 770 to810 nm are known to have COD levels of about 200 mW in their MQW activelayer with no window layer, FIG. 13 graphically shows how much the CODlevel of the window layer is expected to improve with regard to givenblue shift amounts.

[0063]FIG. 13 shows that for samples A, B and C, the blue shift amount λbl and the COD level Pcod are in linear relation to each other.

[0064] A straight line A (broken line in FIG. 13) representing samples Ais plotted when

Pcod=1.3λbl+178.7  (4)

[0065] A straight line B (two-dot chain line) representing samples B isprovided when

Pcod=5.6λbl+50.3  (5)

[0066] A straight line C (dashed line) representing samples C is drawnwhen

Pcod=1.3λbl+153.0  (6)

[0067] A straight line D shown on the left-hand side of the straightline B is formed when

Pcod=5.6 λbl+85  (7)

[0068] A straight line E indicated on the right-hand side of thestraight line C is given when

Pcod=1.3λbl+135  (8)

[0069] In FIG. 13, the points of measurement are shown distributed in anarea enclosed by, the straight lines D and E, covering widely scatteredpoints as well.

[0070] It can be seen that semiconductor lasers with their Pcod lessthan 200 mW have the blue shift amount λ bl of about 20 since the MQWactive layer with no window region has the COD level of about 200 mW.For that reason, if the MQW active layer is disordered so as toconstitute the window layer 20 where the blue shift amount λ bl is atleast 20, then the COD level is considered improved.

[0071] In brief, after the MQW active layer 16 is disordered toconstitute the window layer 20 in the vicinity of the resonator facet,the window layer 20 is subjected to irradiation of pumped light so thatwavelengths of photo luminescence from the window layer 20 are measured.The blue shift amount λ bl is obtained by taking into account the PLwavelength of the MQW active layer with no window region. If the blueshift amount λ bl, turns out to be 20 or greater, it can be concludedthat the COD level is improved at least by formation of the window layer20.

[0072] In the region of FIG. 13 enclosed by the straight lines D and E,the blue shift amount λ bl and the COD level Pcod are considered to bein linearly proportional relation even regarding different samplesfabricated under different conditions.

[0073] For semiconductor lasers at least with oscillation wavelengths of770 to 810 nm and fabricated under different conditions, it is thusimportant to establish the linearly proportional relation between theblue shift amount λ bl and the COD level Pcod. Before each semiconductorlaser is turned into a final product and halfway through the process ofproducing the window layer 20 by disordering the MQW active layer 16near the resonator facet, pumped light is applied to the window layer 20so that the wavelength of photo luminescence from the window layer 20 ismeasured. When the blue shift amount λ bl is obtained by taking intoaccount the PL wavelength of the MQW active layer yet to be disordered,it is possible to predict the COD level Pcod of the end product.

[0074] When the blue shift amount λ bl is defined, in terms of Pcodbased on the equations (7) and (8) above, as a value meeting thecondition of

(Pcod−85)/5.6≦λbl≦(Pcod−0.135.0)/1.3

[0075] then establishing the linearly proportional relation between theblue shift amount λ bl and the COD level Pcod allows the COD level Pcodto be predicted with a high degree of accuracy. This in turn makes itpossible to fabricate highly reliable semiconductor lasers withconsistently and appreciably limited variations of COD degradation. Byallowing COD degradation levels to be predicted in simplified stepshalfway through the production process, the inventive method permitshigh-yield, low-cost fabrication of dependable semiconductor lasers atconsistently low levels of COD degradation.

[0076] The features and the advantages of the present invention asdescribed above may be summarized as follows.

[0077] According to one aspect of the invention, there is provided asemiconductor laser device with an oscillation wavelength of 770 to 810nm, comprising: a semiconductor substrate of a first conductivity type;a first clad layer of the first conductivity type disposed on thesemiconductor substrate; an active layer of a quantum well structuredisposed on the first clad layer; a first second-clad layer of a secondconductivity type disposed on the active layer; a disordered regionformed near a laser resonator facet by introducing impurities from asurface of the first second-clad layer into the layers including theactive layer on the semiconductor substrate; and an optical waveguideincluding a second second-clad layer of the second conductivity typedisposed on the surface of the first second-clad layer in a manneropposite to the active layer in the disordered region across the firstsecond-clad layer, the optical waveguide extending in a resonatorlengthwise direction; wherein if λ dpl is assumed to denote in nm thewavelength of photo luminescence generated by application of pumpedlight to the disordered region and λ apl to represent in nm thewavelength of photo luminescence generated by application of pumpedlight to the active layer, and if a blue shift amount λ bl in nm isdefined as equal to λ apl−λ dpl, then the blue shift amount λ bl meets acondition of λ bl≧20. When the active layer is disordered so as toconstitute the window layer, the semiconductor laser is considered tohave acquired an improved COD level. This makes it possible to fabricatesemiconductor laser devices with consistently and appreciably limitedvariations of immunity from COD degradation.

[0078] In one preferred structure according to the invention, if Pcod isassumed to demote in mW a COD level of the laser device, then the blueshift amount λ bl in nm further may meet a condition of(Pcod−85)/5.6≦λbl≦(Pcod−135.0)/1.3. With this structure, obtaining theblue shift amount λ bl allows the COD level Pcod of the end product tobe predicted. This makes it possible to fabricate semiconductor laserdevices at low cost with enhanced immunity from COD degradation.

[0079] In another preferred structure according to the invention, thesemiconductor laser device may further comprise insulating filmsdisposed on the first second-clad layer and on sides of the opticalwaveguide but not over a top portion of the optical waveguide.Disordering the active layer to form the window layer in thesemiconductor laser of a simple stripe structure makes up asemiconductor laser device with an improved COD level. This permitsfabrication of semiconductor laser devices of the simple stripestructure with consistently and significantly limited variations ofimmunity from COD degradation.

[0080] In a further preferred structure according to the invention, thesemiconductor laser device may further comprise a current blocking layerof the first conductivity type disposed so as to bury the opticalwaveguide on the first second-clad layer. Disordering the active layerto form the window layer in the semiconductor laser of a buriedstructure also constitutes a semiconductor laser device with an improvedCOD level. This permits fabrication of semiconductor laser devices ofthe buried structure with consistently and considerably limitedvariations: of immunity from COD degradation.

[0081] According to another aspect of the invention, there is provided asemiconductor laser device fabricating method including the steps of:firstly forming a first clad layer of a first conductivity type, anactive layer of a quantum well structure, and a first second-clad layersuccessively on a semiconductor substrate of the first conductivitytype; secondly forming on a surface of the first second-clad layer amask pattern for impurity implantation having an opening in a regionwhere a resonator facet of a semiconductor laser device is expected tobe formed thirdly disordering the active layer near, the resonator facetby introducing impurities with the mask pattern used as a mask; fourthlyapplying pumped light to the disordered region to generate photoluminescence therefrom, and measuring a wavelength of the photoluminescence as a basis for predicting a level of COD degradation;fifthly forming a second second-clad layer of the second conductivitytype on the surface of the first second-clad layer after removing themask pattern for impurity implantation; sixthly forming on a surface ofthe second second-clad layer a stripe-shaped mask pattern in a manneropposed to the disordered active layer across the first and the secondsecond-clad layer, the stripe-shaped mask pattern extending in aresonator lengthwise direction; and seventhly forming an opticalwaveguide including the second second-clad layer with the stripe-shapedmask pattern used as a mask. The inventive method allows levels of CODdegradation to be predicted halfway through the process of semiconductorlaser fabrication. This permits high-yield, low-cost fabrication ofsemiconductor laser devices in simplified steps.

[0082] In one preferred variation of the inventive semiconductor laserdevice fabricating method, if the semiconductor laser device has anoscillation wavelength of 770 to 810 nm; if λ dpl is assumed to denotein nm the wavelength of photo luminescence generated by application ofpumped light to the disordered region and λ apl to represent in nm thewavelength of photo luminescence generated by application of pumpedlight to the active layer; and if a blue shift amount λ bl in nm isdefined as equal to λ apl−λdpl, then the blue shift amount λ bi may meeta condition of λbl≧20 when the fourth step is carried out. Disorderingthe active layer to form the window layer in the semiconductor laserwith the oscillation wavelength of 770 to 810 nm makes up asemiconductor laser device with an improved COD level. This permitshigh-yield, low-cost fabrication of semiconductor laser devices withconsistently and significantly limited variations of immunity from CODdegradation.

[0083] In another preferred variation of the inventive semiconductorlaser device fabricating method, if Pcod is assumed to denote in mW aCOD level of the laser device, then the blue shift amount λ bl in nm mayfurther meet a condition of (Pcod−85)/5.6≦λbl≦(Pcod−135.0)/1.3. Thepreferred method allows levels of COD degradation of end products to bepredicted. This permits high-yield, low-cost fabrication ofsemiconductor laser devices with enhanced immunity from COD degradation.

[0084] While the presently preferred embodiments of the presentinvention have been shown and described. It is to be understood thesedisclosures are for the purpose of illustration and that various changesand modifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A semiconductor laser device with an oscillationwavelength of 770 to 810 nm, comprising: a semiconductor substrate of afirst conductivity type; a first clad layer of the first conductivitytype disposed on said semiconductor substrate; an active layer of aquantum well structure disposed on said first clad layer; a firstsecond-clad layer of a second conductivity type disposed on said activelayer; a disordered region formed near a laser resonator facet byintroducing impurities from a surface of said first second-clad layerinto the layers including said active layer on said semiconductorsubstrate; and an optical waveguide including a second second-clad layerof the second conductivity type disposed on the surface of said firstsecond-clad layer in a manner opposite to said active layer in saiddisordered region across said first second-clad layer, said opticalwaveguide extending in a resonator lengthwise direction; wherein if λdpl, is assumed to denote in nm the wavelength of photo luminescencegenerated by application of pumped light to said disordered region and λapl to represent in nm the wavelength of photo luminescence generated byapplication of pumped light to said active layer, and if a blue shiftamount λ bl in nm is defined as equal to λ apl−λ dpl, then the blueshift amount λ bl meets a condition of λ bl≧20.
 2. A semiconductor laserdevice according to claim 1, wherein, if Pcod is assumed to denote in mWa COD level of the laser device, then the blue shift amount λ bl in nmfurther meets a condition of (Pcod−85)/5.6≦λbl≦(Pcod−135.0)/1.3.
 3. Asemiconductor laser device according to claim 1, further comprisinginsulating films disposed on said first second-clad layer and on sidesof said optical waveguide but not over a top portion of said opticalwaveguide.
 4. A semiconductor laser device according to claim 2, furthercomprising insulating films disposed on said first second-clad layer andon sides of said optical waveguide but not over a top portion of saidoptical waveguide.
 5. A semiconductor laser device according to claim 1,further comprising a current blocking layer of the first conductivitytype disposed so as to bury said optical waveguide on said firstsecond-clad layer.
 6. A semiconductor laser device according to, claim2, further comprising a current blocking layer of the first conductivitytype disposed so as to bury said optical waveguide on said firstsecond-clad layer.
 7. A semiconductor laser device fabricating methodincluding the steps of: firstly forming a first clad layer of a firstconductivity type, an active layer of a quantum well structure, and afirst second-clad layer of a second conductivity type successively on asemiconductor substrate of the first conductivity type; secondly formingon a surface of the first second-clad layer a mask pattern for impurityimplantation having an opening in a region where a resonator facet of asemiconductor laser device is expected to be formed; thirdly disorderingthe active layer near the resonator facet by introducing impurities withthe mask pattern for introducing impurity used as a mask; fourthlyapplying pumped light to the disordered region to generate photoluminescence therefrom, and measuring a wavelength of the photoluminescence as a basis for predicting a level of COD degradation;fifthly forming a second second-clad layer of the second conductivitytype on the surface of said first second-clad layer after removing themask pattern; sixthly forming on a surface of the second second-cladlayer a stripe-shaped mask pattern in a manner opposed to the disorderedactive layer across the first and the second second-clad layer, thestripe-shaped mask pattern extending in a resonator lengthwisedirection; and seventhly forming an optical waveguide including thesecond second-clad layer with the stripe-shaped mask pattern used as amask.
 8. A semiconductor laser device fabricating method according toclaim 7, wherein, if the semiconductor laser device has an oscillationwavelength of 770 to 810 nm; if λ dpl is assumed to denote in nm thewavelength of photo luminescence generated by application of pumpedlight to the disordered region and λ apl to represent in nm thewavelength of photo luminescence generated by application of pumpedlight to the active layer; and if a blue shift amount λ bl in nm isdefined as equal to λ apl−λ dpl, then the blue shift amount λ bl meets acondition of λbl≧20 when said fourth step is carried out.
 9. Asemiconductor laser device fabricating method according to claim 8,wherein, if Pcod is assumed to denote in mW aCOD level of the laserdevice, then the blue shift amount λbl in nm further meets a conditionof (Pcod−85)/5.6≦λbl≦(Pcod−135.0)/1.3.